The KOMA-Script classes have a list that basically does what you want:
\begin{labeling}[<separator>]{<widest label>}
...
\end{labeling}
This evironment can be used with the standard classes, too, by loading the scrextend
package. The list would then maybe look like:
\begin{labeling}[---]{\hspace*{4em}}
\item[$P$] pressure
\item[$\eta$] viscosity
\item[$\lambda(P,T)$] thermal conductivity
\end{labeling}
You can probably make nomencl
use the list and you can definitely customize acro
or glossaries
to use it. Here's a way with acro
:

\documentclass{article}
\usepackage{scrextend}% only if you don't use a KOMA-Script class
\usepackage{acro}
\newenvironment{myacrolist}
{\labeling[---]{\hspace*{4em}}}% choose length for widest label
{\endlabeling}
\acsetup{list-type=myacrolist}
\DeclareAcronym{pressure}{
short = \ensuremath{P} ,
long = pressure ,
sort = pressure
}
\DeclareAcronym{viscosity}{
short = \ensuremath{\eta} ,
long = viscosity ,
sort = viscosity
}
\DeclareAcronym{therm-conduct}{
short = \ensuremath{\lambda(P,T)} ,
long = thermal conductivity ,
sort = conductivity thermal
}
\begin{document}
\acuseall
\printacronyms[name=Nomenclature]
\end{document}